This invention relates to gradient index glass rods that can be used as lenses for applications such as collimation, coupling, focusing and active alignments.
Glasses as gradient index media have drawn much attention in fiber optics. A gradient index lens or GRIN lens is one in which the refractive index changes continuously axially, radially or spherically. Axial gradients have surfaces of constant index that are planes normal to the optical axis. In case of a radial GRIN lens, the most common configuration is one where refractive index is a maximum on the center axis and decreases with distance from the center axis. Such refractive index distributions are best approximated by parabolic functions of the type: EQU N(r)=N.sub.0 [1-{(Ar.sup.2)/2 }] (1)
where N(r) is the refractive index at a point in the lens as a function of r which is the distance of the point from the center of the lens, N.sub.0 the refractive index of the lens on the rod axis and A the lens constant. The radial refractive index distribution makes it feasible to focus a ray trajectory on the lens surface, inside the lens and also outside the lens. This gives additional leeway in designing cheap and efficient optical systems. Further, in a GRIN lens, rays of light follow sinusoidal paths whose trajectory is defined in terms of pitch P. One pitch is equivalent to the light wave traversing one sinusoidal cycle. It is related to the lens constant A, and the lens length L by the following relation:
P=(AL)/(2.pi.) (2)
Some earlier patents on the subject, such as U.S. Pat. Nos. 4,495,298, 3,941,474, 4,495,299 and 4,462,663, have already demonstrated that thallium based silicate glasses are ideal for such GRIN lens fabrication. But the methods of the above patents have drawn glass rods from the melt state, i.e., glasses were melted in platinum pots and rods were drawn through a nozzle at the bottom of the pot. Thallium is known to volatilize rapidly at higher temperatures. The drawing process, which is done at substantially higher temperatures makes process control more difficult.
It is therefore desirable to provide an improved lens fabrication method and improved composition of materials for forming such lens.